This invention relates to a switching power supply apparatus which obtains an insulated DC output from a DC power supply, and in particular relates to a switching power supply apparatus which is capable of reducing the power consumption during standby.
In recent years efforts have been made to reduce the power consumption of electrical equipment, as a measure to alleviate global warming. In particular, there have been numerous proposals of inventions relating to switching power supply apparatuses provided with a standby mode, to supply only the minimum necessary electric power to devices, so as to reduce the power consumed when electrical equipment is not being used (see Japanese Patent Laid-open No. 2002-136125 and Japanese Patent Laid-open No. 2004-088959 (corresponding U.S. Pat. No. 6,903,945, corresponding China Patent CN 1311618C). FIG. 10 shows an example of a switching power supply apparatus having a configuration similar to those of the apparatuses disclosed in Japanese Patent Laid-open No. 2002-136125 and Japanese Patent Laid-open No. 2004-088959 (corresponding U.S. Pat. No. 6,903,945, corresponding China Patent CN 1311618C). The operation is explained referring to FIG. 11.
In standby mode, a comparator 6h compares the feedback signal VFB of the DC output voltage with threshold values Vth(H) and Vth(L), set in advance, and transmits a signal VCMP to a logic circuit 6a such that during intervals in which the feedback signal VFB is below the threshold value Vth(L), the on/off operation of the switching element 2 is stopped, and during intervals in which the feedback signal VFB is above the threshold value Vth(H), the switching element 2 is turned on and off. As a result, through intermittent application of a gate voltage VGS to the switching element 2, the number of switching operations per unit time is reduced and power consumption is reduced, in so-called burst oscillation control.
The power supply VCC for the control circuit 6 is obtained by rectifying and smoothing the voltage generated by auxiliary windings 3b provided in the isolation transformer 3; but because power is not supplied to the control circuit 6 during intervals in which switching element 2 on/off operation is halted, the power supply voltage VCC of the control circuit 6 drops depending on power consumption of the control circuit 6. If during this period the DC output voltage falls between the preset voltage, and the feedback voltage VFB exceeds the threshold value Vth(H), switching element 2 on/off operation is resumed, so that the DC output voltage rises and the power supply voltage VCC of the control circuit 6 also rises, and burst oscillation operation is continued.
FIG. 12 is another example of a switching power supply apparatus, with a configuration similar to that disclosed in Japanese Patent Laid-open No. 2000-270546. In the circuit of FIG. 12, when in standby mode, by turning the switch 17 off by means of a control signal cont1 and switching the power supply of the microcomputer 15 to power from the secondary windings 3c of the isolation transformer 3, current is no longer supplied to the photodiode 8a, and the phototransistor 8b turns off. As a result, the supply of power from the auxiliary windings 3b of the isolation transformer 3 to the control circuit 6 is halted, and so power is supplied to the control circuit 6 from the startup circuit 6g. 
At this time the capacitor 5 connected to the power supply of the control circuit 6 is charged, and when the power supply voltage VCC rises to a prescribed voltage, the timer circuit 6d again halts the supply of power from the startup circuit 6g. Then, the voltage across the capacitor 5 falls due to consumption of power by the control circuit 6. When the power supply voltage VCC falls to a prescribed voltage, the timer circuit 6d again starts the supply of power from the startup circuit 6g. By means of this operation, the power supply voltage VCC of the control circuit assumes a triangular wave shape which rises and falls with a prescribed voltage amplitude. The timer circuit 6d counts these triangular-wave periods, and for example controls the logic circuit 6a every two periods to apply the output of the pulse width modulation (PWM) circuit 6b to the switching element 2. Burst oscillation operation is continued by employing the discharge interval of the capacitor 5, that is, the falling interval of the triangular wave, as the period for controlling the switching element 2.
In FIG. 10, a smoothing capacitor 5 with a large static capacitance is connected such that, under conditions in which the DC output power falls below the power consumption of the control circuit 6, such as for example with no load present, the control power supply voltage VCC falls during the interval in which the level of the feedback signal VFB is lower than the threshold value Vth(H), and does not fall below the minimum operating voltage Vuv of the control circuit 6. A capacitor with a large static capacitance is physically large, and so impedes efforts to reduce the size of the switching power apparatus, and also leads to cost increases.
Further, in the circuit of FIG. 12, the power consumption of the startup circuit is the product of the voltage difference between the voltage of the DC power supply 1 and the control power supply voltage VCC, and the current flowing in the startup circuit, and tends to cause an increase in the standby power consumption. Moreover, there is also the problem that when the power supplied to the microcomputer 15 is switched by the switch 17, voltage fluctuations occur at the instant of switching, so that erroneous operation of the microcomputer 15 or similar may occur. Also, when the switching element 2 begins switching, the voltage on capacitor CS gradually rises and the pulse width of the switching element 2 gradually broadens in soft-start operation, so that sharp changes in the current flowing in the isolation transformer 3 are suppressed, and audible sounds generated by the isolation transformer 3 are reduced. However, when the switching element 2 halts oscillation, there is a sudden change from a state in which current is flowing in the isolation transformer 3 to a state in which no current flows, and so there is the problem that there is less effect in decreasing audible noise.
Hence, it would be desirable to reduce the static capacitance of the smoothing capacitor connected to the control circuit, enabling use of a small and low-cost smoothing capacitor, to reduce power consumption, and to reduce audible noises generated by the transformer.